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Sulfuric Acid

Introduction

Of all the heavy industrial chemicals, sulfuric acid is perhaps the most

fundamentally important, in that it has a number of large-scale uses not only

within the chemical industry but in other industries as well.

By far the most important user is the phosphate fertilizer industry.

Other important applications of sulfuric acid are found in petroleum refining,

pigment production, steel pickling, nonferrous metals extraction, and the

manufacture of explosives, detergents, plastics, and man-made fibers.

Many specialty areas of the chemical industry also use varying amounts of

sulfuric acid including the production of dyes, pharmaceuticals, and fluorine

chemicals.

Properties

Physical Properties

Pure sulfuric acid H2SO4, Mw 98.08, is a colorless, water-white, slightly

viscous liquid, melting point 10.4 ◦C, boiling point 279.6

◦C, and density

1.8356.

It can be mixed with water in any ratio.

BACHLEOR OF SCIENCE

SEMESTER-III

PAPER NO.: US03CICH01 (3 CREDITS, 7O MARKS)

(HEAVY AND FINE CHEMICALS)

Unit-1:

Acids: Manufacture, Properties and uses of Nitric acid, Sulphuric acid,

Phosphoric acid, Hydrochloric acid

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Aqueous sulfuric acid solutions are defined by their H2SO4 content in

weight-percent terms. Anhydrous (100 %) sulfuric acid is even today

sometimes referred to as “monohydrate,” which simply means that it is the

monohydrate of sulfur trioxide.

Sulfuric acid will dissolve any quantity of SO3, forming oleum (“fuming

sulfuric acid”).

The physical properties of sulfuric acid and oleum are dependent on the

H2SO4 and SO3 concentrations, the temperature, and the pressure.

Chemical Properties

1. Dehydrating agent –

Sulphuric acid has a great affinity for water and the reaction is extremely

exothermic.

A large amount of heat is produce on mixing acid with water. So in

preparing dilute solutions of H2SO4 the acid should be added to water slowly

with constant stirring. Never add water to the acid.

The evolution of heat is due to the formation of mono and di hydrates

H2SO4.H2O and H2SO4.2H2O.

The formation of these hydrates also explains the dehydrating nature of

sulphuric acid. Therefore it is used for drying almost all gases, except NH3

and H2S.

Its corrosive action on skin is also due to dehydration of skin which then

burns and produces itching sensation.

Due to dehydrating property, it chars sugar to give carbon.

C12H22O11 12C + 11H2O

Similarly, paper, starch, wood etc. are charred by cone. H2SO4 due to the

removal of water. It is also used in removing water from various substances

such as oxalic acid and formic acid.

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COOH-COOH H20 + CO + CO2

2. Oxidising agent

Since sulphuric acid gives O2 on strong heating, hot cone. H2S04 also acts as

an oxidising agent.

3. Pickling agent

Pickling is the process in which layers of basic oxides are removed before

electroplating, enamelling, galvanising and soldering. This is done by

sulphuric acid.

4. Acidic nature

It is a strong dibasic acid and forms two series of salts with alkalies. These

are bisulphates (HSO4- ) and sulphates (SO4

-2).

Manufacture

The manufacture of sulphuric acid is carried out by two processes:

1. The Lead Chamber process or by 2. The Contact process.

The Lead Chamber process.

The Lead Chamber process for the manufacture of sulphuric acid dates back about

200 years. Although less efficient than the contact process, it is still of considerable

commercial importance.

THEORY

Sulphur dioxide is obtained by burning sulphur or roasting pyrites.

S + O2 SO2

4FeS2 + 11O2 2Fe2O3+ 8SO2

The sulphur dioxide thus formed is then oxidised by oxides of nitrogen to

sulphur trioxide. The latter then reacts with steam to produce sulphuric acid.

SO2 + NO2 SO3 + NO

SO3 + H20 H2SO4

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Nitric oxide obtained takes oxygen from air and gets oxidized back to

nitrogen dioxide.

NO + O2 2NO2

In this manner, oxides of nitrogen take oxygen from air and pass it to

sulphur dioxide and act as oxygen carrier.

The main principle of the process is that the moist sulphur dioxide is

oxidized by the oxygen of the air into sulphur trioxide in the presence of

oxides of nitrogen acting as catalyst. Sulphur trioxide thus formed,

combined with water to form sulphuric acid.

The Contact Process for Sulfuric Acid

Essentially all sulfuric acid today is manufactured by the contact process.

A review of the contact process is presented by Phillips and Bland (1984).

The raw material for this procedure is elemental sulfur, sulfur dioxide, or

pyrite (FeS2).

The sources of these materials are as follows:

1. Sulfur from mines.

2. Sulfur or hydrogen sulfide recovered from petroleum

desulfurization.

3. Recovery of sulfur dioxide from coal or oil-burning public utility

stack gases.

4. Recovery of sulfur dioxide from the smelting of metal sulfide ores:

2PbS + 3O2 2PbO + 2SO2

5. Isolation of S02 from pyrite.

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Steps in the Contact Process

As the steps in this process are as follow.

1. Burning of sulfur

S + O2 SO2 (vessel 1)

2. Catalytic oxidation of S02 to S03 (vessel 2)

2SO2 + O2 2SO3

3. Hydration of SO3

SO3 + H2O H2SO4 (vessels 3A and 3B).

Figure: - Flow chart for sulfuric acid manufacture by the contact process. Piping

between catalyst layers from 2A to 2B, and also to 3rd unit (3) not shown.

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Process Details

Step 1. Dry SO2 is needed for the catalytic oxidation. With sources 1 and 2

above, dry air is used in the combustion process. Water will cause

corrosion because of acid formation. If sulfur contains carbonaceous

impurities, the molten material has to be filtered to avoid poisoning the

catalyst and forming water from burning hydrogen.

Step 2. When using sulfur from sources 1 and 2, purification of the SO2 gas is

normally not needed. Other sources of SO2 require wet scrubbing

followed by treatment of the gas with electrostatic precipitators to

remove fine particles. The catalyst used is vanadium pentoxide and the

pressure is 1.2-1.5 atmospheres. The temperature has to be kept around

410-430°C. If it rises above 430°C, the equilibrium is displaced away

from SO3. The value has to reach around 410°C for the catalyst to be

activated. This process is strongly exothermic. Consequently, the

catalytic reactor (2A) is designed as a four-stage fixed-bed unit. The gas

has to be cooled between each step. Four passes, together with "double

absorption," described below, are necessary for overall conversion of

99.5-99.8% (three passes, 97-98%). The temperature rises to over 600°

with the passage of the gas through each catalyst bed. The doubled

absorption consists of cooling the gases between each bed back to the

desired range by sending them through the heat exchanger (2B), and

then back through the succeeding beds. Between the third and fourth

beds, the gases are cooled and sent to an absorption tower like unit (3).

This is not shown on diagram. This is to shift the equilibrium to the

right by absorbing S03. The gases are then sent to the heat exchanger to

warm them to 410-430° and then on to the fourth catalyst bed.

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Step 3. After the catalytic oxidation process, the resulting S03 is hydrated by

absorption in packed towers filled with 98-99% sulfuric acid. This is

the H2S04 azeotrope of minimum total vapor pressure. The catalytic

oxidation has to proceed in high yield to avoid air pollution problems.

S02 has a low solubility in 98% H2S04. At lower acid concentrations,

sulfuric acid and S03 form a troublesome mist and at higher

concentrations emissions of S03 and H2S04 vapor become significant.

The absorption acid concentration is kept within the desired range by

exchange as needed between the H2S04 in the drying acid vessel that

precedes the combustion chamber with the H2S04 in the absorption

tower. The acid strength can be adjusted by controlling the streams of

H2S04 to give acid of 91 to 100% H2S04 with various amounts of added

S03 and water. The conversion of sulfur to acid is over 99.5%.

Uses of Sulfuric Acid

The largest single consumer of sulfuric acid by far is the fertilizer industry.

Most goes into the production of phosphoric acid, which in turn is used to

manufacture such fertilizer materials as triple superphosphate and mono- and

diammonium phosphates.

Lesser amounts are used for producing superphosphate and ammonium

sulfate. About 60 % of the sulfuric acid produced is utilized in fertilizer

manufacture.

Sulfuric acid are used as an acidic dehydrating reaction medium in organic

chemical and petrochemical processes involving such reactions as nitration,

condensation, and dehydration, as well as in oil refining, in which it is used

for refining, alkylation, and purification of crude-oil distillates.

In the inorganic chemical industry sulfuric acid is used notably in the

production of TiO2 pigments, hydrochloric acid, and hydrofluoric acid.

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In the metal processing industry, sulfuric acid is used for pickling and

descaling steel, for leaching copper, uranium, and vanadium ores in

hydrometallurgical ore processing, and in the preparation of electrolytic

baths for nonferrous-metal purification and plating.

Certain wood pulping processes in the paper industry require sulfuric acid,

as do some textile and chemical fiber processes and leather tanning.

Nitric Acid

Introduction

Nitric acid is a strong acid that occurs in nature only in the form of nitrate

salts.

Nitric acid is the most important and useful oxyacid of nitrogen.

It is of great commercial importance as it is used in the manufacture of

various fertilizers (ammonium nitrate and phosphate, nitrophosphate),

explosives (TNT nitroglycerine, cellulose polynitrate, ammonium picrate

etc.), fibres, plastics and dyestuffs.

A growing and potentially very important use of nitric acid is the

replacement of sulphuric acid in acidulation of phosphate rock.

Nitric acid was known to the ancient Egyptians because of its special ability

to separate gold and silver.

Physical Properties.

Nitric acid Mw 63.013 is miscible with water in all proportions.

Pure nitric acid is colourless, fuming and pungent smelling liquid.

The impure nitric acid is yellow due to dissolved oxides of nitrogen, mainly

NO2.

It has a corrosive action on skin and causes painful blisters.

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Chemical Properties.

Acidic properties. Nitric acid is a strong monobasic acid and ionisation in

aqueous solution.

Oxidising properties. It acts as a powerful oxidising agent, due to the

formation of nascent oxygen.

Action on metals. Nitric acid reacts with almost all the metals, except noble

metals, like Pt and Au. The metals are oxidized to their corresponding

positive metal ions while HN03 is reduced to NO, N02. N20, NH2OH or NH3,

depending upon the conditions such as temperature, nature of metal and

concentration of the acid.

Industrial Production

Nitric acid is made by the oxidation of ammonia, using platinum or

platinum-10% rhodium as catalyst, followed by the reaction of the resulting

nitrogen oxides with water. The industrial production of nitric acid by the Ostwald

process is described in this section. The process involves three chemical steps.

1) Catalytic oxidation of ammonia with atmospheric oxygen to yield nitrogen

monoxide:

4NH3 + 5 O2 4 NO + 6H2O

2) Oxidation of the nitrogen monoxide product to nitrogen dioxide or dinitrogen

tetroxide:

2 NO+O2 2NO2 + N2O4

3) Absorption of the nitrogen oxides to yield nitric acid:

3NO2 + H2O 2 HNO3+ NO

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FIGURE Flow chart for nitric acid manufactured by oxidation of ammonia.

Brief Process Description

1. Into the ammonia-air mixer is fed vaporized filtered liquid ammonia and

preheated filtered compressed air. This mixture is led to the converter.

2. The converter contains the catalyst gauzes. This is where reaction (1)

occurs. As shown in Figure the gases leaving the converter are NO with

enough oxygen for reaction (2). as well as nitrogen from the air and from

reaction (6). and water mainly from reaction (l) (temperature. 920°. pressure.

1-4 atm). This is to compress gases because of equipment size limitations.

Pressure does not affect the reaction as there is little volume change; see Eq.(1).

The gases are cooled.

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3. The oxidation of NO to NO2 takes place after the gases leave the cooler,

as well as the absorption in water to give nitric acid. Note the line from the

bottom of the condenser for dilute nitric acid. This is from the water from

reaction (1) with the NO2.

4. The absorption tower has enough water so that 60% nitric acid will result,

and also enough air to convert all remaining NO to NO2. The pressure is 10

atm. to give more efficient absorption.

A more concentrated acid is obtained by distillation. The yield is 94-95%

nitric acid. The product is 60% HNO3. An acid of up to 65% strength can be

obtained if the absorbing system is cooled to 2°C.

Uses

The principal use of nitric acid is as a starting material in the manufacture of

nitrogen fertilizers large amounts are reacted with ammonia to yield

ammonium nitrate.

Weak acid (ca. 60 % HNO3) is most suitable for this purpose. Smaller

amounts of weak acid are used to digest crude phosphates.

About 75 – 85 % of the nitric acid produced is used in the fertilizer sector.

In addition, porous ammonium nitrate prills are still an important

component of explosives.

Nitric acid is used as a nitrating agent in the preparation of explosives and

organic intermediates such as nitroalkanes and nitroaromatics.

It is also used in the production of adipic acid.

Other applications include use as a chemical in metallurgy and in rocket fuel

production

.

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Hydrochloric Acid

Hydrochloric acid, also known as muriatic acid, is a solution of hydrogen chloride

(HCl) in water. Hydrogen chloride (HCl) exists in solid, liquid, and gaseous states

and is very soluble in water.

Properties

Physical Properties

HCl gas is a colourless, pungent smelling gas with acidic taste.

It fumes in moist air and is extremely soluble in water. One volume of water

dissolves 452 volumess of the gas at room temperature.

It is heavier than air.

With water it gives a constant boiling mixture (containing 20.2% of the acid

and boiling at 382K). Dilute solutions of the acid cannot, therefore, be

concentrated by boiling beyond 20.2 %.

It can be easily liquefied to a colourless liquid (b.p 189 K) and frozen to

white crystalline solid. (m.p. 162 K).

Chemical Properties

Hydrogen chloride is thermally stable up to approximately 1500 ◦C, but

appreciable dissociation occurs above this temperature.

Completely dry hydrogen chloride is not very reactive: mild steel is not

measurably attacked. Moreover, reactions involving dry hydrogen chloride

often take place only in the presence of catalysts.

In contrast, a solution of hydrogen chloride in a polar solvent is a strong acid

and, therefore, an aggressive reagent.

Manuctaure of HCl acid

Hydrogen chloride is produced by the direct reaction of hydrogen and chlorine, by

reaction of metal chlorides and acids, and as a by-product from many chemical

manufacturing processes such as chlorinated hydrocarbons.

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1. Synthesis from Hydrogen and Chlorine. Less than 10% of HCl is made

by the direct reaction of the elements. There is large demand in the market

for water white acid. Such acid is obtained by synthetic method and most of

the plants are based on this process.

H2 + Cl2 2 HCl + 184 KJ

Figure show the flow sheet of a synthetic hydrochloric acid plant.

The plant consists of combustion chamber of structural carbon

provided with cooling device which may consist even of cold water

circulation in the shell. The gases are always kept above dew point to avoid

corrosion. The hydrogen and chlorine gas enter the bottom in such

proportions that slight excess of hydrogen is maintained. The reaction is

started by igniting the burner with an external air-hydrogen torch. The HCl

gas is conveyed to the absorber through connection pipe. With almost pure

Burner

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HCl acid gas (98 per cent), the absorption is rapid and external cooling may

be employed. The strength of acid produced is generally 32-33 per cen.

2. Decomposition of Metal Chlorides by Acids. Two commercial processes

employing the acidic decomposition of metal chlorides are the salt–sulfuric

acid process.

3. The Salt–Sulfuric Acid Process. The reaction between NaCl and sulfuric

acid occurs in two stages, both endothermic.

NaCl + H2SO4 NaHSO4 + HCl

NaCl + NaHSO4 Na2SO4 + HCl

4. Hydrogen Chloride as By-Product from Chemical Processes. Over 90%

of the hydrogen chloride produced in the United States is a by-product from

various chemical processes. The crude HCl generated in these processes is

generally contaminated with impurities such as unreacted chlorine, organics,

chlorinated organics, and entrained catalyst particles. A wide variety of

techniques are employed to treat these HCl streams to obtain either

anhydrous HCl or hydrochloric acid. Some of the processes in which HCl is

produced as a by-product are the manufacture of chlorofluorohydrocarbons,

manufacture of aliphatic and aromatic hydrocarbons, production of high

surface area silica, and the manufacture of phosphoric acid and esters of

phosphoric acid.

5. Hydrogen Chloride Produced from Incineration of Waste Organics.

Environmental regulations regarding the disposal of chlorine-containing

organic wastes has motivated the development of technologies for burning

or pyrolyzing the waste organics and recovering the chlorine values as

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hydrogen chloride. Several catalytic and non-catalytic processes have been

developed to treat these wastes to produce hydrogen chloride.

6. Hydrogen Chloride from Hydrochloric Acid Solutions. Gaseous

hydrogen chloride is obtained by partially stripping concentrated

hydrochloric acid using an absorber–desorber system. The stripper is

operated at a pressure of 100–200 kPa (1–2 atm) for improved recovery of

HCl. The overhead vapors consisting of 97% HCl and 3% H2O are cooled to

remove most of the water as concentrated HCl, and the residual water vapor

is removed by drying the gas with sulfuric acid.

Uses

Hydrogen chloride and the aqueous solution, muriatic acid, find application in

many industries.

In general, anhydrous HCl is consumed for its chlorine value, whereas

aqueous hydrochloric acid is often utilized as a nonoxidizing acid.

It is used in metal cleaning operations, chemical manufacturing, petroleum

well activation, and in the production of food and synthetic rubber.

It is used for the manufacture of chlorine and chlorides, eg. Ammonium

chloride used in dry cell.

In the manufacture of glucose from corn starch.

For extracting glue from bones and purifying boneblack.

A saturated solution of zinc chloride in dilute HCl is used for cleaning

metals before soldering or plating.

As a constituent of aqua regia which is used for dissolving noble metals.

It is also used in medicine and as a reagent in the laboratory.